The present invention relates to an arrangement for forming a multilayered paper or paperboard from stock suspensions with papermaking fibres and fines, said arrangement including at least a first and a second forming unit, at least one of the forming units being of twin wire type and having a forming roll for foaming a fibrous web having a higher content of fines u one side than at the other, and a couch device for couching together said fibrous web having a higher content of fines at one side with an other fibrous web to form the multilayered web.
The invention relates to a method of a multilayered paper or paperboard web from stock suspensions with papermaking fibres and fines, said method including the steps of:
forming a first fiber web having a higher content of fineand at one side than at the other in a first forming unit of twin wire type and having a forming roll,
forming a second fiber web in a second forming unit, and
couching the formed webs together to form the multilayered web.
The invention is particularly advantageously implemented when forming testliner, fluting, liner, white-top liner, gypsum board liner, and paperboard based on recycled pulp fibres. However, the invention also can be implemented when forming other paper or paperboard webs.
When manufacturing multilayer liner and paperboard, it is previously known to form bottom layer and top layer(s) for example in so-called fourdrinier forming units, or alternatively in twin-wire forming units of a paper or paperboard machine, in order to subsequently couch the layers together into a coherent web, which is pressed in a press section and dried in a drying section.
It is also previously known that forming the top and bottom layers in fourdrinier forming units, as a rule, results in a finished product with a higher internal bond strength, i.e. bond strength between the layers, than what is achievable if the top and bottom layers arc formed in twin-wire forming units.
The patent publication DE 2 020 688 discloseand a method for joining two webs of paper, paperboard or the like, more specifically a couchinig method, and a device for implementing the method. The method comprises that the two weband ran in the same direction during their manufacture and are joined in a such a way that the wire-side of the secondary web comes to rest on the top side of the primary web.
The device disclosed in DE 2 020 688 comprises a movable long wire (fourdrinier) as a primary wire, and at least one wire arranged in circulation above the long wire as a secondary wire. In the lowest portion of its path, each wire runs in the opposite direction in relation to the long wire, wherein each wire in circulation has been supplied with a transfer roll which is arranged so that it contacts the wire in circulation on an outer side and the long wire on its top side. Thereby, according to DE 2 020 688, the advantage is obtained that always one surface of a paper web manufactured through the method will exhibit the properties of a sheet top-side, such as an improved printability in comparison with a wire-side, wherein this is also the case if more than two paper webs are joined by means of the method.
The patent publication DE 40 31 038 A 1 discloses a device for manufacturing a multilayer paper or multilayer paperboard by means of a primary wire, and a secondary wire arranged above this as twin-wire forming unit, which forms a top layer of the multilayer paper or paperboard onto a bottom layer. Thereby, the twin-wire forming unit has a forming roll as a breast roll for configurating the twin-wires. Upstream the forming roll, a pressure application unit is arranged, wherein the waterline of the pulp suspension is within the region of the forming roll. According to DE 40 31 038 A 1, amongst other things, the disclosed device makes it possible to obtain a higher internal bond strength, since the dry content of the top layer is relatively low when the couching to the bottom layer takes place.
Furthermore, the patent publication U.S. Pat. No. 5,607,555 discloses a forming section of a paper machine having two twin-wire forming units for forming a multilayer web. Each twin-wire forming unit consists of a headbox, two wire loops arranged to define a common wire path for molding the web being formed in a sandwich-like manner, and drainage elements for each of the wire loops. The common wire path of each forming unit has a first section including a curved suction drainage element or roll in the first lower wire loop, and a second section including opposing drainage ledges. Thereby, the drainage ledges are stationary at the upper side and designed to be resiliently pressable on the bottom side. The drainage ledges of the upper and lower sides are arranged staggered with respect to each other in the direction of travel of the wire, wherein at least the ledges of the upper side are designed for suction. Furthermore, there is at least one third section which has at least one suction wire-separating element on one side. The twin-wire forming units are arnnnged so that tee same drainage elements are associated in each case with what will be the inner and the outer sides of each layer in the multilayer web. According to U.S. Pat. No. 5.607,555, a very uniform paper is obtained in this way. Furthermore, its is reported that the contents of filler and/or fines can be displaced towards the outsides of the layers of the paper web by means of arranging the drainage ledges in a suitable way, depending on the prevailing requirements. As a result of this, a larger number of fibres are believed to be present on the two sides of the layers which rest against each other, wherein a better adhesion between the layers is obtained when they have been couched together.
However, it has been found that multilayered paper or paperboard which has been formed in the previously known arrangements can have an insufficient internal bond strength in the z-direction.
One reason for this is that many of the previously known arrangements utilise drainage elements of such types which generate very high shear forces in the pulp suspensions which are dewatcrred Examples of such drainage elements are vactiuum forming rolls and suction boxes with drainage foils generating a high vacuum. High shear forces in the pulp suspensions result in a low retention of the fine material (fines) which is important in order to obtain a high internal bond strength, or at least result in the fines content being strongly reduced at the side of a paper or paperboard laye which has been facing such a drainage element during the dewatering. This often makes it difficult to obtain a sufficient internal bond strength of the finished paper or the finished paperboard.
Accordingly, a first object of the present invention is to provide an arrangement for forming a multilayered paper or paperboard web, which arrangement maxinises the amount of fines which is retained when forming the layers included in the web, and ensures that those sides of the layers which have the highest content of fines are couched together with each other, so that the multilayered paper or paperboard web obtains a raised internal bond strength.
In accordance with claim 1, this first object of the present invention is achieved in that both of the forming units are of twin wire type and include a forming roll for forming a fibrous web having a higher content of fines at one side than at the other, and that the two forming ae so arranged relative to each other as to place the web sides having the highest content of fines in contact with each other for couching together to form the multilayered web.
According to a preferred embodiment of the invention, the forming rolls are open and adapted for making the content of fines in the fibre webs higher at the inner-wire side than at the outer wire-side during the initial dewatering, wherein the additonal drainage members are adapted for maintaining the content of fines at least substantially unchanged at a high level at the inner-wire side during the continued dewatering, and the forming units are designed and arranged in relation to each other so that the two formed fibre webs are joined with the inner-wire sides against each other.
A second object of the present invention is to provide a method for forming a multilayered paper or paperboard web which method maximises the amount of fines that is retained when forming the layers included in the web, and ensures that those sides of the layers that have the highest content of fines are couched together with each other, so that the multilayer paper or paperboard web obtains a raised internal bond strength.
In accordance with claim 14, this second object of the present invention is achieved by:
using as second forming unit a twin wire forming unit having a forming roll,
forming also the second fiber web with a higher content of fines at one side than at the other, and
positioning the webs so as to ensure that those sides of the webs, which have the highest content of fines, are couched together to form the multilayered web.
According to a preferred embodiment of the invention, the method additionally includes the steps of chosing open forming rolls for the initial dewatering, adapting them so that the content of fines in the fibrous webs becomes at least as high at the inner wire side as at the outer wire aide during the initial dewatering, adapting the additional drainage members so that the content of fines will be maintained at least substantially unchanged at a high level at the inner wire side during ghe continued dewatering, and joining the two formed fibrous webs with the inner wire sides against each other.
It should be noted that the expression xe2x80x9copen forming rollsxe2x80x9d, as used herein, means that a two-sided dewatering on the forming rolls, if possible, is accomplished without the application of any external suction.
When the wire speed of a twin-wire forming unit is increased, as is well-known to the skilled person, a stage will be reached where the g-force becomes so large that the inwardly directed pressure generated by the tension in the outer-wire no longer is able to press any water through the inner-wire, so that all dewatering will take place through the outer-wire. In such cases, it may become necessary to generate a negative pressure within the forming roll in order to be able to direct at least part of the dewatering through the inner-wire. An entirely symmetrical dewatering would be possible to achieve only in cases where there were no g-forces at all.
For this reason, the expression xe2x80x9copen forming rollsxe2x80x9d, as used herein, is also intended to include embodiments of the invention where a suction forming roll with a low, or at the most, a moderate negative pressure is utilised in order to prevent all dewatering from taking place through the outer-wire at high wire speeds, but in which embodiments the main drainage impulse still is generated by the wire tension during the passage of the wires over the curved peripheral surface of the forming roll.
Further objects of the present invention will become evident from the following description, while the features which enables the further objects to be achieved are defined in the dependent claims.